Despite the fact that the benefits of regular physical activity for preventing chronic disease and alleviating mental health problems are widely publicized, approximately 40% of American adults participate in no leisure-time physical activity. According to the text of the Healthy People 2010 program, "starting out slowly with an activity that is enjoyable ... is central to the adoption and maintenance of physical activity behavior". Yet, the hypothesized causal pathway from the intensity of physical activity, to the affective responses to physical activity (e.g., enjoyment), and, finally, to adherence has yet to be studied in its entirety. To lay the foundation for this research, the purpose of the proposed exploratory study is to collect data on the first link, namely the relationship between the intensity of physical activity and affect in a demographic group (adults between 35 and 55 years) with a high prevalence of physical inactivity. The specific aims of the study are to examine (a) whether the transition from aerobic to anaerobic metabolism is the turning point toward affective decline during physical activity, (b) what level of intensity of physical activity previously sedentary adults self-select and what affective responses they report, and (c) whether the responses to a prescribed ("imposed") level of physical intensity that exceeds one's self-selected level by a very small margin (10% of maximal aerobic capacity) has a significant negative impact on affect. In addition, the study will (a) test the prediction of the "dual-mode" theory (Ekkekakis, 2003) that physical self-efficacy is the primary correlate of affect near the point of transition from aerobic to anaerobic metabolism and metabolic strain (indexed by the respiratory exchange ratio) becomes the primary correlate near the point of maximal capacity and (b) examine whether individual differences in somatosensory modulation and self-efficacy can predict the level of self-selected physical activity intensity, the affective responses to physical activity performed at a self-selected intensity, and the affective responses to physical activity of prescribed intensity. The data will be collected from 60 healthy but previously sedentary men and women based on a series of physical activity sessions in the laboratory. The immediate anticipated benefit of this research is the development of physical activity public health recommendations that incorporate affect as a fundamental consideration. [unreadable] [unreadable] [unreadable]